Post-accelerated color-kinescopes



June 11, 1957 D. w. EPSTEIN ETAL 2,795,720

POST-ACCELERATED COLOR-KINESCOPES Filed Se pt. 15, 1955 INVEN TORS DAV/o I'M 5675/ 4 1 97 TOR/V6 V POST-ACCELERATED COLOR-KENESCOPES David W. Epstein, Princeton, N. 5., and Edward G. Ramberg, Upper Southampton Township, Bucks County, Pa., assignors to Radio Corporation of America, a corporation of Delaware Application September 15, 1955, Serial No. 534,491

1 Claim. (Cl. 313-85) This invention relates to cathode-ray tubes for producing television pictures in full color and, has special reference to improvements in color-kinescopes of the focased-mask line-screen variety.

An object of the invention is to construct a novel and improved focused-mask cathode-ray tube or kinescope of the line-screen type which insures improved picture quality.

Stated generally, this object is achieved in accordance with the invention by the provision of a cathode-ray tube wherein the mask wires are supported by a curved frame so that the mask surface has a cylindrical shape, and the phosphor line screen has a cylindrical curvature which is less than that of the mask and is so related to the latter as to insure more uniform separation of the color lines in a plane through the tube axis.

Various other objects and advantages will be apparent as the nature of the invention is more fully disclosed.

French Patent 866,065 to Dr. Werner Flechsig (published June 16, 1941) and its German equivalent 736,575

(published June 22, 1943) describe a color kinescope n containing a bi-part screen assembly which comprises (i) a transparent viewing screen having a target surface made up of a multiplicity of groups of parallelly arranged strips or lines of difierent (e. g. red, blue and green) coloremissive areas and (ii) a grill formed of a large number of spaced-apart wires mounted adjacent to the target side of said screen. The beam electrons travel in substantially straight paths through the space between the grill and the screen. These straight beam-paths terminate, ideally, on respectively difierent ones of the (red, blue and green) line-like screen areas. The proper geometrical location of the grill wires provides a Crookesrshadow which masks those line-like screen areas which at any given instant are to remain unilluminated.

One very real limitation upon the eflicient' use of the I above-described Crookes-shadow tubes is that only a, small percentage of the beam electrons ever reach the viewing screen; the larger percentage being dissipated, in the form of heat, upon impact with the imperforate parts of the grill or mask. This limitation was recognized by Dr. Flechsig and his patents mention that it can be mini mized (a) by making the mask wires of such fine gauge that the Crookes-shadow is no longer a factor in the operation of the tube, and (b) by substituting for the Crookesshadow etfect a beam-focusing cylindrical-lens effect which he achieved by operating the viewing screen at a relatively high potential with respect to the fine wire grill.

The present invention relates to post-accelerated cathode-ray tubes of the above-described lenticular grill variety and employs a cylindrically curved mask and a cylindrically curved screen which are so designed and constructed, in accordance with the invention, as to achieve uniform separation of the color lines in a plane through the tube axis which is at right angles to the mask wires; reduce defocusing at the edges and corners of the picture;

States Patent Patented June 11, 1957 the scanning beams than has heretofore been attained with tubes of this type employing either fiat masks or curved masks and screens of conventional construction.

The invention is described in connection with the ac companying single sheet of drawings, in which:

Fig. 1 is a diagrammatic view, in longitudinal section, illustrating a focusing-mask kinescope of the line-screen type embodying the invention; and

Fig. 2 is a diagram which will be referred to in explaining the determination of the screen shape which, for a given grill radius, will result in the proper spacing of the phosphor lines in a plane perpendicular to the wires through the tube axis.

The color-kinescope diagrammatically illustrated in Fig. l is of the line-screen variety disclosed in the above-mentioned French and German Flechsig patents,

and comprises an evacuated envelope 10 having a main chamber 12 in the form of a frustum which terminates at its large end in a cylindrically curved window 13 which constitutes the transparent foundation plate of the target or screen 14 of the kinescope.

The target surface 14 of the window 13 is provided with a multiplicity of parallelly disposed phosphor lines of different color-emissive characteristics, usually red, blue and green, arrangedin a repetive pattern in groups of three. An electron-transparent light-reflecting film constituted, for example, of evaporated aluminum, coats said phosphor lines and renders the entire target surface of the screen conductive.

The cylindrically curved focusing mask or grill 15 mounted within the main chamber 12 of envelope 10 adjacent the screen 14 is formed of a large number of spaced-apart wires stretched between suitable cylindrically curved frames, in a manner well understood in the art.

The small end of the frustum 12 terminates in a tubular neck 16 which contains the usual beam source of electrons and the beam-focusing and deflecting means employed for scanning the target assembly.

A preferred embodiment of the tube employs a battery of three electron guns 17 in a plane, with a separation such that the distance between efiective beam centers measured in a direction at right angles to the phosphor lines in the deflection plane is are. (The deflection-plane in a multi-beam C. R. tube is the planein whichthe axis of each deflected beam, when extended rearwardly from the screen-unit, intersects the axis of origin of that beam.) The plane of the guns 17 is at right angles to the mask wires located on the cylindrical surface, of radius Rg, of the mask 15. The mask wires, with a mutual separation d, may be electrically connected to the tube cone or conductive coating on the tube walls. mask 15 may be. made slightly negative with respect to the tube Walls, or to a high-transparency additional grid which may be placed between the guns and the mask. Establishing such a small voltage between the mask and a the tube walls reduces the number of secondary elecand in general to achieve a more eflicieut utilizationof trons drawn from the mask to the screen and thus'improves picture quality. 7

The screen 13 is maintained at a voltage, measured with respect to the gun cathodes, which is approximately four times that of the masklS'. It is given a; cylindrical curvature which is less than that of the mask. Formulas for arriving at the preferred radius of curvature, which leads to uniform separation of the color lines in a plane through the tube axis which is at right angles to the mask wires, are given below. For example, if theradius of curvature of the mask is made 1.5 times its distance L from the plane of deflection and the;s.eparation between mask and screen is a =0.07 3 L, the optimal screen curvature lies in the range from 1.8 L to. l.9 L.

As an alternative, the.

The fact that the beam has a smaller angle of incidence on the curved mask than it would have on a flat mask leads to smaller defocusing with deflection. Even for a tube with a total horizontal deflection angle of 80 and a vertical deflection angle of 60, the defocusing in the corners is less than a color line when the center is focused sharply. Furthermore, the deviation from straightness of the pattern lines is only of the ,order of one color line and the variation in thespacing of adjoining different-color lines is less than 8 percent. These specificnumerical values refer to the foregoing example of the tube with Rg: 1.5 L, a =0.073 L. 1

The geometrical relationships of the focusing-mask tube are indicated in Figs. 1 and 2. The tube employs the single wire grill 15 at ultor potential Twith a radius of curvature Rg and-the cylindrically curved screen 13 of radius R at a distance a (measured perpendicularly to the grill) from it. The potential V of the screen 13 is assumed to be 4Vthe distance L: 13.695" between center grill and deflection plane being treated as very large compared to the distance a. In the numerical examples the value of a at the center, 0 :1".

The objectives of the calculations given are the following:

l. The determination of the screen shape (i. e. the value of R5 or the variation of a with the deflection angle 0) which, for given grill radius Rg, will result in uniform spacing of the phosphor lines in a plane perpendicular to the wires through the tube axis.

2. A comparison of the spacing of adjoining phosphor lines in the corner of the picture and at the vertical edge center (wires vertical).

3. Determination of the deviation from straightness of the phosphor lines excited by the central gun (guns in line).

4. Determination of maximum width of the lines formed by the lens action of the grill in a horizontal and in a diagonal direction.

Basic formuIas.For the derivation of the relevant expressions it is convenient to introduce the following quantities:

fl =arc sin X ,/L* deflection angle projected on plane .5. to phosphor lines (horizontal plane) R angle of incidence of ray from center of deflection plane on grill, projected on plane .L to phosphor lines 0,,=a.rc tan y,/L* deflection angle projected on plane I} to phosphor lines L*=-./R,(R L) sin 0,,(R,L) cos 8 distance from center of deflection plane to point of incidence on grill projected on plane perpendicular to phosphor lines X ,,*=X for 0,,=0 (horizontal section through tube axis) a distance between grill and screen (measured along normal to grill) q=2a tan 6'/(1+X,,*) displacement at screen of prin- 9=arc sin sin 9,]

cipal ray from center of deflection plane relative to 7 measured at right angle to grill normal.

line of each trio is given by l X l X,,*)

Finally, the line broadening at the picture edge or at the picture corners for a sharply focused center is given by Znx with A:r,= {(X='=1)(1-1-tan 6)cos e'-1} If the defocusing at the center and in the corners is balanced and Ax is very much smaller than w =M d/3, Ax for the corners indicates also the maximum line width (for vanishing beam cross section in the deflection plane) throughout the picture.

d is the separation of the wires of the grill, D, their thickness.

Calculations Were carried out in detail for Rg/L=1 and Rg/L=l.5 with L=l3.695 and a =l". This corresponds to a pattern magnification M =l.0489 and a beam separation in the deflection plane Xc=0.251" if d=0.035. D is assumed to be 0.003". With increasing value of a the screen is flattened increasingly (Rs increased) as compared with the grill. Apart from this the value of a as Well as those of Mo, d, and x have no appreciable effect on the values given below.

0, 8, R, (tel/dew tic-[we Arm/w Art/w,

' 0, 9, R, xix-Idem 614w, Am/wn Art/wt As a matter of orientation, changing 0 from 1" to 2" changes Rs for 0 =O from 15.76 to 17.99".

Discussion.'I'he angles in the first two columns are the assumed maximum deflection angles in a horizontal (lateral sides) and vertical direction (top and bottom). R3 is the radius of curvature of the screen which leads to equal spacing of color lines at the centers of the sides and in the middle of the picture. The fourth column gives the ratio of the spacing of adjoining color lines at the top and at the horizontal center line of the picture, for equal horizontal deflection. The fifth column gives the deviation from straightness of the center lines of a color line trio, measured in terms of the width We of a phosphor line at the center. The last two columns indicate the line broadening at the horizontal center line and in the corners measured in the same units.

It is seen that for an overall angle of 72 (0=30) and Rg=1.5 L, Rs=25" the deviation from straightness is still less than half a color line, the defocusing of the order of )6 of a color line, and the variation in the spacing of adjoining color lines only about 4 percent. Electrons reflected by the screen will tend to introduce halos and decrease contrast. On the other hand, secondary electrons from the grill can be largely collected by an added high-transparency grill slightly positive with respect to the focusing mask.

Among the more important advantages of a focusingmask kinescope of the line-screen type constructed in accordance with the present invention may be cited the following: It has the high mask transparency characteristic of a focusing-mask tube; it is possible to deposit the screen directly on a face plate of reasonable thickness, since the curvature insures high mechanical strength under pressure; ease of printing the screen, since the phosphor lines closely approximate straight lines parallel to the cylinder axis; reduced defocusing at the edges and corners of the picture; and the possibility of making line spacing uniform by giving the screen the proper curvature.

What is claimed is:

A cathode ray tube for producing pictures in full color, comprising in combination: a plurality of electron beam producing guns arranged adjacent to each other, deflecting and accelerating means common to said guns and arranged to deflect the electron beams therefrom at a common deflecting plane perpendicular to the tube axis, a cylindrically curved fluorescent screen disposed in the paths of said beams and upon which said pictures are produced, said screen having groups of lines of color phosphors thereon extending along lines parallel to the axis of curvature of said cylindrical screen, and a focusing grill interposed between said guns and said screen, said grill being formed of laterally spaced wires extending parallel to said cylindrical axis, the longitudinal axes of said wires lying in a common cylindrical surface having a radius of curvature substantially one and one-half times the maximum distance from said grill to said deflecting plane and the radius of curvature of said screen being of the order of from 1.8 to 1.9 times said maximum distance, the cylindrical axes of said screen and said grill lying in a common plane through the tube axis and being at right angles to the tube axis, the minimum spacing between said screen and said grill being substantially 0.073 times said maximum distance from said grill to said deflecting plane.

References Cited in the file of this patent UNITED STATES PATENTS 2,745,978 Van Ormer May 15, 1956 FOREIGN PATENTS 866,065 France Mar. 31, 1941 

